This invention relates to fuel cells and, in particular, to frame assemblies and manifolds for use in fuel cell stacks.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series to form a fuel cell stack.
A fuel cell stack may be an internally manifolded stack or an externally manifolded stack. An internally manifolded stack typically includes gas passages for delivery of fuel and oxidant built into the fuel cell plates. In an externally manifolded stack, fuel cell plates are left open on their ends and gas is delivered to the cells by way of manifolds sealed to the respective faces of the fuel cell stack. The manifolds in each type of fuel cell stack provide sealed passages for delivery of fuel and oxidant gases to the fuel cells and prevent those gases from leaking to the environment and to the other manifolds. These functions of the manifolds must be performed under the operating conditions of the fuel cell stack and for the duration of the stack life.
A gas seal established between the manifold edge and the fuel cell stack face is an important aspect of manifold performance. The manifolds, which are made from metallic materials, must be electrically isolated from the fuel cell stack which is typically electrically conductive and has an electrical potential gradient along its length and from its edge to the manifold. Dielectric insulators have been used between the metallic manifold and the fuel cell stack to produce electrical isolation between the manifold and the stack and to prevent the manifold from shorting out the stack, and to control electromotively driven electrolyte migration from the stack to the manifold. Dielectric insulators are typically made from brittle ceramic or mica materials which may be damaged as a result of thermal and mechanical stresses or chemical reaction on the manifold system during fuel cell operation. Moreover, a fuel cell stack will usually shrink over its life because the cell components creep and densify at high temperature and pressure. Such shrinkage and changes in fuel cell stack dimensions create additional stresses on the manifold system during stack operation. Accordingly, there is a need for a manifold system capable of absorbing at least partially the thermal and mechanical stresses so as to prevent damage to the manifold sealing system dielectric insulators and to effectively maintain electrical isolation and electrolyte migration inhibition between the manifold and the stack.
Modifications to manifold system components have been developed to improve the flexibility of the components and to better accommodate thermal expansion and contraction of the stack and the mechanical stresses on the dielectric insulator resulting therefrom. Conventional dielectric insulators currently used in the manifold system typically comprise a number of interlocking segments which allow the insulator to expand and contract with the stack. An example of such a dielectric insulator arrangement is disclosed in U.S. Pat. No. 4,414,294. In particular, the '294 patent discloses a rectangular insulator having a plurality of segments interconnected by slidable spline joints. Each of these spline joints includes a spline key fitted into keyways formed by adjacent end portions of the insulator segments. As shown and described in the '294 patent, the keys fitted into the keyways conform to the shape of the keyways and extend across the thickness of the keyways to provide sufficient gas sealing.
In addition, flexible manifolds have also been developed to improve their gas sealing capabilities and to reduce dielectric insulator damage. U.S. application Ser. No. 10/264,866 assigned to the same assignee herein discloses an example of such a flexible manifold system. Particularly, the '866 application discloses a manifold system having a manifold body which includes a number of pan sections connected by compound folds and a rail assembly including a number of rail members with a plurality of kerfs cut through a portion of their thickness at positions corresponding to the compound folds of the manifold body. The folds and the kerfs of the manifold system of the '866 application lend flexibility to the manifold assembly and improve the gas seal established between the manifold edge and the stack face. The manifold body and the rail assembly in the '866 application are electrically isolated from the stack by dielectric insulators which are secured to the rail assembly using round or cylindrical stainless steel pins protruding from the rail members.
One of the common problems associated with the above arrangements is the fracturing or breaking of the ceramic electrolyte migration inhibitor and electric insulator in joint areas of the dielectric insulator, i.e. the areas around the keyways and the pinning holes. These fractures and breaks result from the keys and pins reacting on the dielectric insulator to the resultant mechanical stresses and applied forces during the expansion and contraction of the stack.
Another common problem is the tearing and erosion of gaskets abutting the dielectric insulator caused by the sharp edges of the insulator keyway areas and of the keys. Such fractures, breaks and erosion may result in breaking of the gas seal and a breach of electrical isolation between the manifold system and the stack. Accordingly, means of minimizing stress on the dielectric insulator in the joint areas, and particularly in the keyway areas and in the areas around the pins, are needed to reduce dielectric failure and to redirect the mechanical forces away from the insulator during stack operation.
It is therefore an object of the present invention to provide a dielectric frame member having interlocking segments designed to overcome the above disadvantages.
It is also an object of the present invention to provide a dielectric frame member which is adapted to prevent tearing and erosion of the abutting gaskets.
It is a further object of the present invention to provide a fuel cell stack manifold having an improved pin design which reduces the mechanical stresses on the dielectric frame member.